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1 .TH PCREPARTIAL 3 "24 February 2012" "PCRE 8.31"
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "PARTIAL MATCHING IN PCRE"
5 .rs
6 .sp
7 In normal use of PCRE, if the subject string that is passed to a matching
8 function matches as far as it goes, but is too short to match the entire
9 pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might
10 be helpful to distinguish this case from other cases in which there is no
11 match.
12 .P
13 Consider, for example, an application where a human is required to type in data
14 for a field with specific formatting requirements. An example might be a date
15 in the form \fIddmmmyy\fP, defined by this pattern:
16 .sp
17 ^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
18 .sp
19 If the application sees the user's keystrokes one by one, and can check that
20 what has been typed so far is potentially valid, it is able to raise an error
21 as soon as a mistake is made, by beeping and not reflecting the character that
22 has been typed, for example. This immediate feedback is likely to be a better
23 user interface than a check that is delayed until the entire string has been
24 entered. Partial matching can also be useful when the subject string is very
25 long and is not all available at once.
26 .P
27 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
28 PCRE_PARTIAL_HARD options, which can be set when calling any of the matching
29 functions. For backwards compatibility, PCRE_PARTIAL is a synonym for
30 PCRE_PARTIAL_SOFT. The essential difference between the two options is whether
31 or not a partial match is preferred to an alternative complete match, though
32 the details differ between the two types of matching function. If both options
33 are set, PCRE_PARTIAL_HARD takes precedence.
34 .P
35 If you want to use partial matching with just-in-time optimized code, you must
36 call \fBpcre_study()\fP or \fBpcre16_study()\fP with one or both of these
37 options:
38 .sp
39 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
40 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
41 .sp
42 PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-partial
43 matches on the same pattern. If the appropriate JIT study mode has not been set
44 for a match, the interpretive matching code is used.
45 .P
46 Setting a partial matching option disables two of PCRE's standard
47 optimizations. PCRE remembers the last literal data unit in a pattern, and
48 abandons matching immediately if it is not present in the subject string. This
49 optimization cannot be used for a subject string that might match only
50 partially. If the pattern was studied, PCRE knows the minimum length of a
51 matching string, and does not bother to run the matching function on shorter
52 strings. This optimization is also disabled for partial matching.
53 .
54 .
55 .SH "PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()"
56 .rs
57 .sp
58 A partial match occurs during a call to \fBpcre_exec()\fP or
59 \fBpcre16_exec()\fP when the end of the subject string is reached successfully,
60 but matching cannot continue because more characters are needed. However, at
61 least one character in the subject must have been inspected. This character
62 need not form part of the final matched string; lookbehind assertions and the
63 \eK escape sequence provide ways of inspecting characters before the start of a
64 matched substring. The requirement for inspecting at least one character exists
65 because an empty string can always be matched; without such a restriction there
66 would always be a partial match of an empty string at the end of the subject.
67 .P
68 If there are at least two slots in the offsets vector when a partial match is
69 returned, the first slot is set to the offset of the earliest character that
70 was inspected. For convenience, the second offset points to the end of the
71 subject so that a substring can easily be identified.
72 .P
73 For the majority of patterns, the first offset identifies the start of the
74 partially matched string. However, for patterns that contain lookbehind
75 assertions, or \eK, or begin with \eb or \eB, earlier characters have been
76 inspected while carrying out the match. For example:
77 .sp
78 /(?<=abc)123/
79 .sp
80 This pattern matches "123", but only if it is preceded by "abc". If the subject
81 string is "xyzabc12", the offsets after a partial match are for the substring
82 "abc12", because all these characters are needed if another match is tried
83 with extra characters added to the subject.
84 .P
85 What happens when a partial match is identified depends on which of the two
86 partial matching options are set.
87 .
88 .
89 .SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()"
90 .rs
91 .sp
92 If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre16_exec()\fP
93 identifies a partial match, the partial match is remembered, but matching
94 continues as normal, and other alternatives in the pattern are tried. If no
95 complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
96 PCRE_ERROR_NOMATCH.
97 .P
98 This option is "soft" because it prefers a complete match over a partial match.
99 All the various matching items in a pattern behave as if the subject string is
100 potentially complete. For example, \ez, \eZ, and $ match at the end of the
101 subject, as normal, and for \eb and \eB the end of the subject is treated as a
102 non-alphanumeric.
103 .P
104 If there is more than one partial match, the first one that was found provides
105 the data that is returned. Consider this pattern:
106 .sp
107 /123\ew+X|dogY/
108 .sp
109 If this is matched against the subject string "abc123dog", both
110 alternatives fail to match, but the end of the subject is reached during
111 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
112 identifying "123dog" as the first partial match that was found. (In this
113 example, there are two partial matches, because "dog" on its own partially
114 matches the second alternative.)
115 .
116 .
117 .SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()"
118 .rs
119 .sp
120 If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre16_exec()\fP,
121 PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
122 continuing to search for possible complete matches. This option is "hard"
123 because it prefers an earlier partial match over a later complete match. For
124 this reason, the assumption is made that the end of the supplied subject string
125 may not be the true end of the available data, and so, if \ez, \eZ, \eb, \eB,
126 or $ are encountered at the end of the subject, the result is
127 PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
128 been inspected.
129 .P
130 Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
131 subject strings are checked for validity. Normally, an invalid sequence
132 causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
133 special case of a truncated character at the end of the subject,
134 PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when
135 PCRE_PARTIAL_HARD is set.
136 .
137 .
138 .SS "Comparing hard and soft partial matching"
139 .rs
140 .sp
141 The difference between the two partial matching options can be illustrated by a
142 pattern such as:
143 .sp
144 /dog(sbody)?/
145 .sp
146 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
147 longer string if possible). If it is matched against the string "dog" with
148 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
149 PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
150 if the pattern is made ungreedy the result is different:
151 .sp
152 /dog(sbody)??/
153 .sp
154 In this case the result is always a complete match because that is found first,
155 and matching never continues after finding a complete match. It might be easier
156 to follow this explanation by thinking of the two patterns like this:
157 .sp
158 /dog(sbody)?/ is the same as /dogsbody|dog/
159 /dog(sbody)??/ is the same as /dog|dogsbody/
160 .sp
161 The second pattern will never match "dogsbody", because it will always find the
162 shorter match first.
163 .
164 .
165 .SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()"
166 .rs
167 .sp
168 The DFA functions move along the subject string character by character, without
169 backtracking, searching for all possible matches simultaneously. If the end of
170 the subject is reached before the end of the pattern, there is the possibility
171 of a partial match, again provided that at least one character has been
172 inspected.
173 .P
174 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
175 have been no complete matches. Otherwise, the complete matches are returned.
176 However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
177 complete matches. The portion of the string that was inspected when the longest
178 partial match was found is set as the first matching string, provided there are
179 at least two slots in the offsets vector.
180 .P
181 Because the DFA functions always search for all possible matches, and there is
182 no difference between greedy and ungreedy repetition, their behaviour is
183 different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
184 the string "dog" matched against the ungreedy pattern shown above:
185 .sp
186 /dog(sbody)??/
187 .sp
188 Whereas the standard functions stop as soon as they find the complete match for
189 "dog", the DFA functions also find the partial match for "dogsbody", and so
190 return that when PCRE_PARTIAL_HARD is set.
191 .
192 .
193 .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
194 .rs
195 .sp
196 If a pattern ends with one of sequences \eb or \eB, which test for word
197 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
198 results. Consider this pattern:
199 .sp
200 /\ebcat\eb/
201 .sp
202 This matches "cat", provided there is a word boundary at either end. If the
203 subject string is "the cat", the comparison of the final "t" with a following
204 character cannot take place, so a partial match is found. However, normal
205 matching carries on, and \eb matches at the end of the subject when the last
206 character is a letter, so a complete match is found. The result, therefore, is
207 \fInot\fP PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
208 PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
209 .
210 .
211 .SH "FORMERLY RESTRICTED PATTERNS"
212 .rs
213 .sp
214 For releases of PCRE prior to 8.00, because of the way certain internal
215 optimizations were implemented in the \fBpcre_exec()\fP function, the
216 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
217 all patterns. From release 8.00 onwards, the restrictions no longer apply, and
218 partial matching with can be requested for any pattern.
219 .P
220 Items that were formerly restricted were repeated single characters and
221 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
222 conform to the restrictions, \fBpcre_exec()\fP returned the error code
223 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
224 PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
225 pattern can be used for partial matching now always returns 1.
226 .
227 .
228 .SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
229 .rs
230 .sp
231 If the escape sequence \eP is present in a \fBpcretest\fP data line, the
232 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
233 that uses the date example quoted above:
234 .sp
235 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
236 data> 25jun04\eP
237 0: 25jun04
238 1: jun
239 data> 25dec3\eP
240 Partial match: 23dec3
241 data> 3ju\eP
242 Partial match: 3ju
243 data> 3juj\eP
244 No match
245 data> j\eP
246 No match
247 .sp
248 The first data string is matched completely, so \fBpcretest\fP shows the
249 matched substrings. The remaining four strings do not match the complete
250 pattern, but the first two are partial matches. Similar output is obtained
251 if DFA matching is used.
252 .P
253 If the escape sequence \eP is present more than once in a \fBpcretest\fP data
254 line, the PCRE_PARTIAL_HARD option is set for the match.
255 .
256 .
257 .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()"
258 .rs
259 .sp
260 When a partial match has been found using a DFA matching function, it is
261 possible to continue the match by providing additional subject data and calling
262 the function again with the same compiled regular expression, this time setting
263 the PCRE_DFA_RESTART option. You must pass the same working space as before,
264 because this is where details of the previous partial match are stored. Here is
265 an example using \fBpcretest\fP, using the \eR escape sequence to set the
266 PCRE_DFA_RESTART option (\eD specifies the use of the DFA matching function):
267 .sp
268 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
269 data> 23ja\eP\eD
270 Partial match: 23ja
271 data> n05\eR\eD
272 0: n05
273 .sp
274 The first call has "23ja" as the subject, and requests partial matching; the
275 second call has "n05" as the subject for the continued (restarted) match.
276 Notice that when the match is complete, only the last part is shown; PCRE does
277 not retain the previously partially-matched string. It is up to the calling
278 program to do that if it needs to.
279 .P
280 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
281 PCRE_DFA_RESTART to continue partial matching over multiple segments. This
282 facility can be used to pass very long subject strings to the DFA matching
283 functions.
284 .
285 .
286 .SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()"
287 .rs
288 .sp
289 From release 8.00, the standard matching functions can also be used to do
290 multi-segment matching. Unlike the DFA functions, it is not possible to
291 restart the previous match with a new segment of data. Instead, new data must
292 be added to the previous subject string, and the entire match re-run, starting
293 from the point where the partial match occurred. Earlier data can be discarded.
294 .P
295 It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
296 treat the end of a segment as the end of the subject when matching \ez, \eZ,
297 \eb, \eB, and $. Consider an unanchored pattern that matches dates:
298 .sp
299 re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
300 data> The date is 23ja\eP\eP
301 Partial match: 23ja
302 .sp
303 At this stage, an application could discard the text preceding "23ja", add on
304 text from the next segment, and call the matching function again. Unlike the
305 DFA matching functions, the entire matching string must always be available,
306 and the complete matching process occurs for each call, so more memory and more
307 processing time is needed.
308 .P
309 \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
310 with \eb or \eB, the string that is returned for a partial match includes
311 characters that precede the partially matched string itself, because these must
312 be retained when adding on more characters for a subsequent matching attempt.
313 However, in some cases you may need to retain even earlier characters, as
314 discussed in the next section.
315 .
316 .
317 .SH "ISSUES WITH MULTI-SEGMENT MATCHING"
318 .rs
319 .sp
320 Certain types of pattern may give problems with multi-segment matching,
321 whichever matching function is used.
322 .P
323 1. If the pattern contains a test for the beginning of a line, you need to pass
324 the PCRE_NOTBOL option when the subject string for any call does start at the
325 beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
326 doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
327 includes the effect of PCRE_NOTEOL.
328 .P
329 2. Lookbehind assertions that have already been obeyed are catered for in the
330 offsets that are returned for a partial match. However a lookbehind assertion
331 later in the pattern could require even earlier characters to be inspected. You
332 can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the
333 \fBpcre_fullinfo()\fP or \fBpcre16_fullinfo()\fP functions to obtain the length
334 of the largest lookbehind in the pattern. This length is given in characters,
335 not bytes. If you always retain at least that many characters before the
336 partially matched string, all should be well. (Of course, near the start of the
337 subject, fewer characters may be present; in that case all characters should be
338 retained.)
339 .P
340 3. Because a partial match must always contain at least one character, what
341 might be considered a partial match of an empty string actually gives a "no
342 match" result. For example:
343 .sp
344 re> /c(?<=abc)x/
345 data> ab\eP
346 No match
347 .sp
348 If the next segment begins "cx", a match should be found, but this will only
349 happen if characters from the previous segment are retained. For this reason, a
350 "no match" result should be interpreted as "partial match of an empty string"
351 when the pattern contains lookbehinds.
352 .P
353 4. Matching a subject string that is split into multiple segments may not
354 always produce exactly the same result as matching over one single long string,
355 especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
356 Word Boundaries" above describes an issue that arises if the pattern ends with
357 \eb or \eB. Another kind of difference may occur when there are multiple
358 matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
359 is given only when there are no completed matches. This means that as soon as
360 the shortest match has been found, continuation to a new subject segment is no
361 longer possible. Consider again this \fBpcretest\fP example:
362 .sp
363 re> /dog(sbody)?/
364 data> dogsb\eP
365 0: dog
366 data> do\eP\eD
367 Partial match: do
368 data> gsb\eR\eP\eD
369 0: g
370 data> dogsbody\eD
371 0: dogsbody
372 1: dog
373 .sp
374 The first data line passes the string "dogsb" to a standard matching function,
375 setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
376 for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
377 string "dog" is a complete match. Similarly, when the subject is presented to
378 a DFA matching function in several parts ("do" and "gsb" being the first two)
379 the match stops when "dog" has been found, and it is not possible to continue.
380 On the other hand, if "dogsbody" is presented as a single string, a DFA
381 matching function finds both matches.
382 .P
383 Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
384 multi-segment data. The example above then behaves differently:
385 .sp
386 re> /dog(sbody)?/
387 data> dogsb\eP\eP
388 Partial match: dogsb
389 data> do\eP\eD
390 Partial match: do
391 data> gsb\eR\eP\eP\eD
392 Partial match: gsb
393 .sp
394 5. Patterns that contain alternatives at the top level which do not all start
395 with the same pattern item may not work as expected when PCRE_DFA_RESTART is
396 used. For example, consider this pattern:
397 .sp
398 1234|3789
399 .sp
400 If the first part of the subject is "ABC123", a partial match of the first
401 alternative is found at offset 3. There is no partial match for the second
402 alternative, because such a match does not start at the same point in the
403 subject string. Attempting to continue with the string "7890" does not yield a
404 match because only those alternatives that match at one point in the subject
405 are remembered. The problem arises because the start of the second alternative
406 matches within the first alternative. There is no problem with anchored
407 patterns or patterns such as:
408 .sp
409 1234|ABCD
410 .sp
411 where no string can be a partial match for both alternatives. This is not a
412 problem if a standard matching function is used, because the entire match has
413 to be rerun each time:
414 .sp
415 re> /1234|3789/
416 data> ABC123\eP\eP
417 Partial match: 123
418 data> 1237890
419 0: 3789
420 .sp
421 Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
422 the entire match can also be used with the DFA matching functions. Another
423 possibility is to work with two buffers. If a partial match at offset \fIn\fP
424 in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
425 the second buffer, you can then try a new match starting at offset \fIn+1\fP in
426 the first buffer.
427 .
428 .
429 .SH AUTHOR
430 .rs
431 .sp
432 .nf
433 Philip Hazel
434 University Computing Service
435 Cambridge CB2 3QH, England.
436 .fi
437 .
438 .
439 .SH REVISION
440 .rs
441 .sp
442 .nf
443 Last updated: 24 February 2012
444 Copyright (c) 1997-2012 University of Cambridge.
445 .fi

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